Light emitting diode (LED) markets have grown based on low-output LEDs used in a portable communication device such as a mobile phone, a keypad of a small-sized home appliance, or a backlight unit of a liquid crystal display (LCD). Recently, the need for high-output and high-efficiency light sources that are used in interior lighting, external lighting, internal and external spaces of a vehicle, and a backlight unit of a large-sized LCD increases, and LED markets have targeted in high-output products.
The biggest problem in LEDs is low light emitting efficiency. In general, light emitting efficiency is determined by efficiency of generating light (internal quantum efficiency), efficiency in which light is emitted to an external space of a device (external light extracting efficiency), and efficiency in which light is converted by phosphor. It is important to improve a characteristic of an active layer in view of internal quantum efficiency so as to produce high-output LEDs. However, it is very important to increase external light extracting efficiency of light that is actually generated.
The biggest hindrance that occurs when light is emitted to an external space of an LED is internal total reflection caused by a difference in refractive indexes between layers of the LED. Due to the difference in refractive indexes between the layers of the LED, about 20% of light generated is emitted to an external space of an interface between the layers of the LED. Furthermore, light that is not emitted to the external space of the interface between the layers of the LED moves within the LED and is changed into heat. As a result, light emitting efficiency is low, and the amount of heat generated in a device is increased, and the life span of the LED is reduced.
In order to improve external light extracting efficiency, a method of increasing the roughness of a p-GaN surface or an n-GaN surface, a method of making the surface of a substrate as a bottom part of the device rough or a method of forming curved patterns has been suggested.
FIG. 1 is a cross-sectional view of an LED 14 formed on a substrate 10 in which patterns 12 are formed, and FIG. 2 is a view of the substrate 10 in which the patterns 12 are formed. In particular, when the patterns 12 are formed on the substrate 10 of the LED 14 using a different substrate such as a sapphire substrate, external light extracting efficiency is improved.
Patterns formed on the surface of a sapphire substrate are calculated to increase external light extracting efficiency by 100% or more. Korean Patent Application No. 2004-0021801 and No. 2004-0049329 disclose shapes of patterns formed on the surface of the sapphire substrate or the patterns. A method of forming the patterns by using etching has been currently used. In the method of forming the patterns by using etching, in order to form semispherical patterns on a sapphire substrate, a thick layer resist having a thickness of several tens of micrometers is patterned, and then the resist and the sapphire substrate are simultaneously etched by dry etching.
In the method of forming the patterns by using etching, the height of the patterns is limited by an etching selectivity between the resist and the substrate, and due to low uniformity of a process of patterning the thick layer resist and a dry etching process, uniformity of patterns that are finally formed is low. First of all, contamination that occurs in dry etching is the biggest problem. Due to heat that is locally generated during etching, a reactant of the resist and gas that is used in etching remains on the surface of the sapphire substrate and is not completely removed even though a cleaning process is performed. In addition, damages may occur in the surface of the substrate due to high energy gas particles used in etching (Silicon processing for the VLSI ear, vol. 1, process technology, p. 574-582). When such contamination occurs, if GaN epitaxial growth as the next process is performed, defects may occur in a nitride epitaxial layer due to contamination. Due to the above disadvantage, when a device is fabricated by using the sapphire substrate that is patterned by using an etching process, a very low yield is expected.
In the above-mentioned dry etching process, in order to emit an excessive amount of heat generated when sapphire is forcibly etched, a high-priced etching equipment having a cooling function should be used. In order to improve light extracting efficiency, a process of reducing the size of patterns etched using a high-priced photographing equipment such as a stepper should be performed. Thus, costs increase when the above-mentioned dry etching process is performed. In addition, in the process in which the photographing equipment such as the stepper is used, process throughput is not easily increased due to a complicated process.